The invention relates to a conveying system for passing web- or sheet-shaped photographic layer-bearing carriers through the photochemical baths of a developing apparatus in which the layer-bearing carriers are passed through between at least one pair of parallel conveying rollers, at least one of the rollers of each pair is a driven one, and the passage gap of a roller pair for the layer-bearing carriers is elastically resiliently enlargeable in a direction perpendicularly to the shafts of the rollers of that pair.
Nowadays, hobby photographers as well as professional photographers do no longer carry out the developing, rinsing and fixing steps of their processing of exposed photographic paper manually, but this is now done with the aid of more or less completely automated developing machines into which the exposed photographic material is introduced and in which it undergoes the various known developing stations. The transportation of the papers through the machine is usually carried out by means of pairs of so-called conveying or transport rollers, wherein the photographic paper is passed by friction through a passage gap between every two conveying rollers being in engagement with one another. At least one of the two rollers of each pair is a driven one, and the passage gap is adapted to be resiliently enlarged to a small degree to be adapted to the thickness of the photographic layer-bearing carrier. The property of the passage gap being resiliently enlargeable is of particular importance in the developing process because an excessively high roller pressure on the light-sensitive layer is liable to have a negative influence on the developing process. On the other hand, a certain minimum of roller pressure is needed as, firstly, a secure guidance of the slippery paper material passing between the conveying rollers must be safeguarded if an obstruction in the movement of paper through the apparatus is to be avoided, and, secondly, a certain squeezing off of the residual potassium bromide solution left on the upper side of the developed layer during the developing process is to take place. Moreover, it is important that roller pressure is always applied substantially uniformly, as, otherwise, there may be obtained streaky differences in the appearance of the developed products.
In the known apparatus this enlargeability of the passage gap has been achieved by different modes of construction:
(a) For a considerable length of time, polyvinyl chloride rollers have been in use as the afore-mentioned conveying rollers and have been satisfactory in certain aspects because of their chemical resistance. However, the manufacture of these rollers is relatively expensive as they require very exact finishing, on account of the hardness of their material, in order to attain a true rotation and to avoid pressure fluctuations between the rollers during rotation.
A resilient support for these hard rollers was achieved by providing at least one roller of each pair of rollers with a slotted hole bearing, and resiliently pressing the two rollers together by means of a coil spring or leaf spring. However, coil springs have the fundamental drawback that, over a certain length of time, residues of developer chemicals will be deposited in the spring bodies so that the springs are progressively prevented from "breathing", i.e. contracting and expanding. Leaf springs have not been found to have a long life because the corroding influence of chemicals in the development baths cause the spring material to lose its elastic properties and become brittle. Bearings using springs have been found to be, in the long run, costly in production, assembly and maintenance.
(b) Rubber rollers have also been in use for some time as conveying rollers. Due to their softness, rubber rollers have the advantage that complicated spring equipment for their bearings is not indispensible. Yet, it has been found that the production of rubber rollers is also not free from problems, as rubber is not as resistant to chemicals as polyvinyl chloride. Indeed, rubber mixtures have been developed in the past which possess adequate chemical resistance to photochemical baths. But even these types of rubber rollers suffer from certain drawbacks, as the resilience of the rubber mixtures in question is limited, and they are subject to a certain swelling process in the development baths which cannot be controlled in a desired manner. This swelling can lead to an increase of the roller diameter of from about one tenth to two tenths of a millimeter, a range which can have a negative influence on the contact pressure between the paper and the roller, and thereby on the developed product, in particular if it is undesirable to make very high demands on the tolerances of the roller bearings.
(c) Other known apparatus use conveying rollers bearing a textile coating which possesses certain resilient properties. This textile coating suffers, however, from a serious drawback, namely, that residues of developer chemicals are deposited in the same, and these residues on the rollers will, in the long run, have a negative influence on the results of the development process. In order to keep this negataive influence as small as possible the textile coating should be replaced within a period of from three to four months, which requires a considerable investment of time in their maintenance; for conveying racks must be completely dismantled to this end.
(d) Finally, there must be mentioned so-called foam rollers used as conveying rollers which have the same negative properties as the textile-coated rollers. A particularly negative influence of the foam rollers does, however, reside in their absorption properties; for, when a rack bearing such rollers is taken out of a bath, a considerable amount of bath solution is removed from the tank containing the same. It may take a whole day for the rollers to dry after they have been taken out of the bath, i.e., until all residual liquid has dripped off from the rollers. Moreover, it is relatively difficult to clean the foam rollers. Last not least, there should be mentioned the high weight of the liquid-saturated rollers which are to be removed together with the rack from the tank. A fully saturated roller is so heavy, weighing some tens of kilograms, that a small gantry crane is needed to lift the rack out of the tank.